Microwave switch



Se t. 28, 1954 J. F. ZALESKI 2,690,539

MICROWAVE SWITCH Filed Oct. 4, 1951 IN VEN TOR.

7 Z Z A BY /0///V fTZ/ulss/a Patented Sept. 28, 1954 MICROWAVE SWITCH John F. Zaleski, Valhalla, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application October 4, 1951, Serial No. 249,710

6 Claims.

This invention pertains to electrical switches for microwave transmission lines.

Electromagnetic energy of frequencies for which hollow wave guides are useful as transmission lines is commonly called microwave energy, and requires control components that usually are quite different from those used at lower frequencies. Even in the simple operation of switching, such as performed in lower frequency circuits by a double pole double throw switch, special facilities are required. For example, in microwave circuits a knife switch cannot be employed because dielectric paths rather than discrete conducting wires transmit the field. energy. Also microwave circuits cannot be broken and left unterrninated and on the other hand when such circuits are connected, mere continuity alone is not enough to establish a connection without consideration of resonance efiects.

Various forms of microwave switches have been heretofore proposed but in general these switches have had the disadvantage of being cumbersome to operate, difiicult to design, requiring special matching provisions to reduce discontinuity effects and likewise such switches are capable of use only in a narrow band of frequencies, 1. e., narrow band components.

The instant invention employs a four-arm E- plane or series junction, in which the axes of four hollow wave guide arms meet in a plane at a single point, with equal 90 angles between adjacent arms. One of the arms is employed as the input terminal pair and the opposite arm serves as a guide matching stub, and is eliectively short-circuited at a length designed to give a selected matching impedance. Two solid stubs are employed to aid the guide matching stub in eliminating reflections. The two remaining arms constitute the output terminals. A dipole tuned to the frequency employed is pivoted in the center of the junction of the four arms, and depending on its orientation the incoming energy is reflected and diverted to one or the other of the outgoing arms. the other output arm being interdicted. The pivoted dipole can of course, be rotated manually, electromagnetically or in any other desired manner.

The wave guides employed in this switch may be either rectangular or circular, preferably employing TE1,: and Tic-1,1 modes of transmission respectively. Other numbers of arms than four can be employed in the microwave switch of this invention, such as three or five arms, with some sacrifice in neutralization of mismatch during switching.

The microwave switch of this invention is broadband and does not require excessive accuracy in any part of its design. The use of a single resonant dipole at the center of the device greatly facilitates arrangements for operating the switch and permits the rotating parts to be made small and with very low inertia. Discontinuities are so well balanced out that during operation of the switch the voltage standing wave ratio in the input arm at no time exceeds approximately 1 When the switch is thrown to one side or the other the interdicted output arm emits only a negligible amount of energy, being approximately 30 db below that of the other output arm.

It is obvious that in general this microwave switch is reversible and that two inputs can be applied to the two output arms with one output being derived from the input arm. However, the substantial absence of reflections and losses during switching above mentioned, will not be se cured when the input and output are reversed unless the two inputs are exactly opposed in phase and equal in magnitude.

This microwave switch can be employed for the simple purpose of opening and closing a single microwave circuit with almost complete absence of transient disturbance by connection of one of the output arms to a resistive and non-reflective load.

The matching arm of this switch can also be employed as an additional microwave terminal instead of being short-circuited, but in such instance its matching function is largely 10st and during switching a larger voltage standing wave ratio is likely to exist in the input branch. When all four arms are so employed the microwave switch serves the purpose of a conventional fourpole double throw switch as employed in lowfrequency switching.

The principal purpose then of this invention is to provide a simple mechanical device for switching microwaves from one microwave circuit to another.

Another purpose of this invention is to provide a microwave switch which is broadband and efficient.

Another purpose of this invention is to provide a microwave switch having such parameters as to lend itself to easy design in any microwave length range.

Another purpose of this invention is to provide a microwave switch which almost completely eliminates momentary high voltage standing wave ratios during switching.

Another purpose of this invention is to provide a microwave switch having a single resonant moving part of exceedingly low inertia.

A further understanding of this invention and its purposes may be secured from the detailed description and the drawings, in which:

Figure 1 is an isometric view of the microwave switch of the invention.

Figure 2 is a side view, partly in section, of the switch of Fig. 1. 1

Figures 3A and 3B are views of one form of tuned switch rotor.

Figure 4 is a sectional view taken on the line 4-4 of Fig. 2.

Referring now to Fig. 1, a rectangular wave guid four-way junction is composed of four arms ll, l2, I3 and I4, meeting at a central point at the axis Iii-46. This juncture is of the type commonly known as an E-plane junction, and any arm considered in relation to the two adjacent arms has an electrical relation thereto similar to the series circuit relation in wire circuits. The guide is of the standard size for the selected operating microwave frequency. For instance, for 9000 megacycle per second operation, rectangular guide 1 inch by A; inch externally with 0.050 inch wall is satisfactory.

Arm l l is here designated as the input arm and serves as the terminal pair for reception of microwave energy applied to the switch. Arms 12 and Hi are similar to each other and may be designated as the output arms. These arms serve as the terminal pairs for alternative transmission of microwave energy from the switch. All three arms ll, [2 and It are preferably terminated in the guide characteristic impedance in accordance with conventional microwave practice.

The arm I3 is closed at its end i! by a solid flat metal plate, thus acting as a short-circuited guide stub. The function of arm I3 is to assist in tuning out or balancing out the discontinuity of the junction during switching, when the switch arm or rotor is at any position intermediate its two switched positions of rest. The design of such a matching stub is well known in the microwave art and its length may be either calculated or arrived at experimentally. In one instance, for example, an appropriate length for the arm l3 was found to be .3125)\ in which )\g, is the wavelength in guide.

A rotor shaft extends through the junction along the line of the axis Iii-l6. This shaft is journalled in two bearings I 8 and I9, each containing a conventional wave trap or labyrinth. The bearings are made massive and extend a slight distance into the interior space of the wave guide junction, where these metallic extensions serve an impedance matching function.

The construction may be more readily understood from Fig. 2, which is a view, partly in section, of the guide junction of Fig. 1 looking directl into the arm [2. The faces of the guide arms H and I3 then extend above and below the arm I? respectively. The rotor shaft 2| extends through bearing holes in the bearings -I9 and 18. A rotary electromagnetic relay solenoid havin 90 throw is depicted at 22 and is secured to and supported by the bearing E8. The relay armature is secured to the shaft 2| so that movements of the armature, when the relay is electrically energized and deenergized through its lead conductors 23, wil1 result in rotational displacement of the shaft 2! through a 90 arc.

Wave guide traps of any conventional design are preferably used at the journals of the shaft 2i. For example, an annular trap 24 is provided internally of the bearing l9 and consists of two quarter-wave sections 20 and 27 in series, so that the journal edge 28 is at a high impedance point and the discontinuity 29 in the giude wall has low impedance.

The bearings l8 and I9 extend in the form of solid metallic plugs or discs 3i and 32 into the interior space of the guide junction, thus having the effect of constricting the dimension d at the center of the junction. This in turn chan es the impedance of the guide space at the junction, and in conjunction with th effect of the matching stub I3 matches the effect of the junction on the input arm. For example, in one instance the junction was found to be effectively matched at 9000 mcs. when the diameter d of each plug was made to be 0.650 inch and the amount e by which each plug extended into the interior space was made to be 0.050 inch.

A rotor dipole 33 is carried by the shaft 2| and is located in the center plane bisecting the broad sides of the four guid arms. In its most effective form the rotor 33 preferably consists simply of a thin rod of circular cross-section secured at its center to the shaft 2| and extending perpendicularly thereto. The length of this dipole rotor is calculated and designed to hvae a length of approximately so that it resonates at exactly the microwave frequenc employed. Such a rotor dipole is very broadband, operating with only slight change of efiiciency over a 10% frequency band. For example, a rotor designed for operation in this junction at 9000 mcs. will operate over the band 8500-9500 mes. with a maximum V. S. W. R. presented to the input terminal I i (Fig. 2) not in excess of 1.10 in either switched position.

The diameter of the dipole rotor rod can be made as little as inch and still retain adequate stiffness and strength, and can be as large as 1% nch without producing undue reflection from its end back into the input arm II when in an intermediate switching position.

Additionally, the dimension of the dipole rotor in the direction parallel to its axis of rotation may be increased to reduce corona discharge and to increase the breakdown voltage. Such a rotor is depicted in Figs. 3A and 3B, in which the thickness t of the rotor 34 is approximately equal to the diameter of the shaft 2|. This design has the additional advantage in reducing voltage breakdown because such a rotor is made slightly shorter, for a given resonant wavelength, than the rod type. Such a rotor, however, is somewhat less broadband and has greater inertia than the rod type.

Obviously a reat many other resonant rotor structures can be employed in place of those described, and may be useful in special cases, although in general they are less broadband and have higher inertia.

In order to accommodate the rotor in the center space of the guide junction this space is drilled out so that clearance of approximately 0.010 inch or more exists between the rotor ends and the nearest parts of the guides. This operation is desirable primarily to make these clearances equal everywhere. This is illustrated in exaggerated form in Fig. 4, representing a crosssectional view of Fig. 2 on the line 44 except that the dipole rotor is shown in switched position. The center space is drilled out on the diameter D. In one example this dimension D was 0.650 inch and equal to the diameters of the matching plugs, while the length of the rotor 33 was 0.630 inch, providing a clearance of 0.010 inch at each of the four corner locations 36, 31, 38 and 39.

When the rotor is at a selected one of its two switched positions and thus nearest to one opposite pair of the corners 36, 31, 38 and 39, capacitance exists between the rotor and the corners. These two capacitances provide a low impedance path completing the wall of the guide, thus reducing the impedance of the guide path from the input arm to the selected output arm. Measurements have established that the resulting loss through a switch designed as indicated herein is substantially the same as that which would be experienced in an equal length of rectangular guide of the same size. Most of the loss that does occur, amounting to a mere 6% of the output, is due to leakage through the output arm.

In operation, microwave energy is applied to the arm I I and the tuned rotor 33, Fig. 4, is set at an angle of 45 so that its ends are adjacent the corners and 38. The microwave energy is then switched to the arm I2 and delivered to the component attached thereto with no more loss than would be caused by a length of rectangular guide replacing the switch. The energy escaping past the rotor and out the output arm It is from 25 to db below the energy level in arm I2. By means of the rotary electromagnetic solenoid 22, Fig. 2, the rotor 33, Fig. 4, is now moved to the position in which its ends are adjacent the corners 31 and 39, causing the microwave energy to be cut off from the output arm I2 and switched to the output arm I4.

In order that mismatch during the switching operation may be reduced to a minimum, the dipole rotor 33 should be moved through the 90 arc indicated at 4| from the solid line to dotted line position. When moved through this particular arc the longitudinal length of the dipole is then maintained more nearly perpendicular to the electrical field in the arm II during its travel from one switch position to the other.

At either switched position discontinuities or losses are negligible. In any intermediate position of the rotor 33, as for example in the position in line with the axis of arm I i, discontinuity may exist because the input energy feeds into both output arms simultaneously, and the effective impedance of both of these arms together is obviously not the same as that of the input arm alone. This discontinuity that is evident during the course of the switching operation is, however, substantially matched out by the short-circuited guide stub I3 and the two plugs 3| and 32, Fig. 2. When the rotor is in either switched position the guide stub I3 plays no part. What part the plugs 3| and 32 play is beneficial as they substantially cancel whatever small discontinuity is introduced by the employment of the tuned rotor to complete the guide path in these positions.

Obviously when the arm I3 is not short-circuited but is connected to a properly matched load, the arm cannot exercise the function of compensation for the discontinuity introduced during switching. However, no change is made in the described operation in either of the switched positions. When arm I3 is so connected, another microwave path is made available, so that in one switched position one path is between arms II and I4 and the other path is between arms I2 and I3, in the other switched position one path is between arms II and I2 and the other path is between arms I3 and I4.

What is claimed is:

l. A microwave switch comprising, four waveguide members connected together in a single E- plane junction in cruciform arrangement with their axes lying in a single plane and intersecting at a common point, a dipole element consisting of a single longitudinal rod positioned with its center coinciding with said common point and its axis lying in the plane of the axes of said waveguide, one of said waveguide members being short circuited at one end the length of said short circuited waveguide member being such as to constitute an impedance matching stub at the operating frequency of said switch, and means for rotating said dipole element about said common point through an arc which com prehends said closed end waveguide member.

2. A microwave switch as set forth in claim 1 having a pair of conductive plugs extending into the junction of said wave guide members on opposite sides of said single plane with the axes of said plugs aligned with said common point.

3. A microwave switch comprising, four rectangular wave guide sections connected together in cruciform arrangement with their axes lying in a single plane and intersecting at a common point and their broad faces perpendicular to said single plane, a dipole element positioned with its center at said common point and its width bisected by said single plane, means for rotating said dipole element about its center in said single plane through an angle which is subtended by the narrow dimension of one of said wave guide sections, the wave guide section in linear alignment with said subtending wave guide section being short circuited at one end thereof, the length of said short circuited waveguide section being such as to constitute an impedance matching wave guide stub at the operating frequency of said switch.

4. A microwave switch as set forth in claim 3 having a pair of conductive lugs extending into said wave guide sections on opposite sides of said single plane with their axes aligned with said common point.

5. A microwave switch comprising, four rectangular waveguide sections connected together in cruciform arrangement with their axes lying in a single plane and intersecting at a common point and their broad faces perpendicular to said single plane, the interior intersecting corners of said waveguide sections being cut away to form a cylindrical recess the axis of which passes through said common point normal to said single plane, one of said waveguide sections being short circuited at one end the length of said short circuited waveguide section being such as to constitute an impedance matching stub at the operating frequency of said switch, an elongated dipole mem'ber positioned with its linear dimension in said plane and its center at said common point, the linear dimension of said dipole element being less than the diameter of said cylindrical recess, and means for rotating said dipole element about its center in said plane through an angle which is subtended by the narrow di- References Cited in the file of this patent UNITED STATES PATENTS Name Date Irving Aug. 30, 1949 Number OTHER REFERENCES Ragan, Microwave Transmission Circuits, Radiation Lab. Series, vol. 9. Copyright 1948, McGraw-Hill Co. Pages 209-214 and 535-537. (Copy in Div. 69.) a 

